The present disclosure is related to semiconductor devices and, more particularly, to the fabrication of thyristor-based semiconductor memory devices with MOSFETs in close proximity to bipolar devices.
When forming a MOSFET in close proximity to a bipolar device, as in the case of forming a thyristor-based memory device, the typical procedures for fabricating the MOSFET may degrade the characteristics of the bipolar device. In other words, implants specific to the formation of the MOSFET may adversely affect the qualities or characteristics of the bipolar device.
Typically, a given mask may protect thyristor regions during implants specific to the MOSFET device; and, likewise, another mask may be used to protect regions of the MOSFET during implants specific to the thyristor device. For large geometry devices, the lateral spacing between the body region for the MOSFET and the base region for the bipolar device may allow mask definition sufficient to protect an emitter region of the bipolar device during implants specific to the MOSFET and vice versa.
However, when forming the MOSFET and bipolar devices in close proximity for thyristor-based memory of increased circuit density and higher level of integration, the tolerance, limitations of typical known process procedures may limit the ability to protect against proximity effects. Process variations in combination with the limited masking resolution may impact the lateral penetration/placement of dopants within the semiconductor material, such that the typical procedures may not be well suited for realization of thyristor-based semiconductor memory devices of reduced geometrics. When scaling the devices down for greater density, the bipolar device may eventually become more vulnerable to an adverse lateral influence of a MOSFET when formed in close proximity thereto using the typically known methods of fabrication. The increased vulnerability may further result in a greater variation in characteristic current gain for the bipolar device, which may become even more pronounced when forming mirror image devices.
For example, left and right thyristor memory devices to a mirror-image pair may become more sensitive to asymmetries associated with the individual processing steps that are employed during their fabrication as a result of the greater proximity of the MOSFETs to associated thyristors. The processes and/or patterning during definition of the left devices for the mirror-image pair will be opposite to the corresponding processes and/or patterning for definition of the right device. Hence for a given shift in patterning and/or processing, the effect may be doubled wherein the left devices may be affected in one way and the mirror image right device in the opposite way. This may lead to greater variation in characteristics from cell to cell.
As recognized by the present disclosure, the advancements for increased integrations with closer MOSFET and bipolar proximities may reach a certain threshold where typical procedures for fabricating thyristor-based memory devices, of commonly-shared emitter and drain/source regions, may no longer be viable for sustaining meaningful manufacturing yields and product reliability.